In the fields of digital audio and telecommunications, among others, it has become increasingly desirable, in converting from digital to analog signals and analog to digital signals, to produce signals of low distortion and noise. Many systems which implement both DACs and analog-to-digital converters (hereinafter "ADCs") commonly perform "digital oversampling" and "analog oversampling", respectively. Digital oversampling is a technique by which there is provided from a digital input signal of a first rate, a corresponding second digital signal of a second rate which is significantly greater than the first rate. By contrast, analog oversampling is a technique by which an analog signal is sampled at greater than its Nyquist rate.
Additionally, many such systems employ sigma-delta modulators. Sigma-delta modulation is a process used, in part, to manipulate the noise spectrum of an input signal so that most of the noise power ("quantization noise") therein is moved to frequencies substantially outside the signal bandwidth. This is referred to as "noise shaping". A filter typically reduces the out of band shaped quantization noise to acceptable levels for handling by external components, such as loudspeakers.
The filter used to reduce the out of band quantization noise is generally a switched-capacitor filter. Switched-capacitor filters employ a network of capacitors and switches to "sample" periodically a reference voltage source, which provides a reference voltage. The "sampling" includes periodically charging one or more capacitors from the reference voltage source. This operation is also referred to as "charge loading".
In a system, such as a mixed-signal integrated circuit, which includes both switched-capacitor DAC and ADC circuits, two DACs are typically synchronously operated from the same system clock; one DAC is the primary DAC in the DAC circuit ("the main DAC") and the other DAC ("the secondary DAC") is connected in the feedback path of the ADC circuit. Problems are encountered when the two synchronous switched-capacitor DACs are operated at different rates. This situation occurs when the main DAC includes control circuitry for altering the clock frequency at which the DAC modulator operates without simultaneously altering the clock frequency of the ADC circuitry by the same value. This situation can also occur if digital input signals of varying rates are input to the main DAC and the rate at which the ADC system operates is independent of the rates of the digital input signals. The application of the same assignee, titled A METHOD FOR VARYING THE INTERPOLATION RATIO OF A DIGITAL OVERSAMPLING DIGITAL-TO-ANALOG CONVERTER SYSTEM AND APPARATUS THEREFOR, filed on even date herewith as Ser. No. 08/043,408 now U.S. Pat. No. 5,313,205, which is hereby incorporated by reference herein, discloses an apparatus and method for controlling the interpolation ratio of an oversampled DAC such that signal quantization noise is shaped substantially outside of the signal bandwidth. Upon a decrease in the input sampling frequency, the interpolation ratio is increased. A corresponding increase in the ADC circuitry (i.e., decimator) does not take place. Thus, in such a system, the two DACS (that is, the filters of the two DACs) operate at different rates when the input sampling frequency is changed.
One problem associated with this situation is that gain errors in the DAC outputs will occur. This is so because the mismatch in the sample rates of the DACs will present a varying load to the reference voltage source. This variation will, in turn, change the effective value of the reference voltage "seen" by the main DAC (which is operating at a different, generally higher rate).
Additionally, a more serious problem results, relating to a shift in the noise spectrum. In particular, because the switched-capacitor filter of the main DAC is operating at a faster rate than the switched-capacitor filter of the secondary DAC, charge sampling of the reference voltage source is occurring at two different rates. The charge sampling output of the faster DAC will be modulated and, in the case of a sigma-delta DAC, "inter-modulation" of the shaped quantization noise will occur. This inter-modulation shifts the quantization noise spectrum into a lower frequency band (in certain applications, within the bandwidth of the signal being processed). This result is highly undesirable and can cause high in-band noise and deterious unwanted idle tones.